The invention relates to a metal-halide lamp comprising a discharge vessel with a ceramic wall, the discharge vessel enclosing a discharge space which contains an ionizable filling which filling contains halides of Na and Tl in addition to Hg.
A lamp of the type defined in the opening paragraph is known from EPA-A-0 215 524. The lamp comprises tungsten electrodes. The known lamp, which combines a high specific luminous flux with excellent color properties (inter alia general color rendition index Raxe2x89xa780 and a color temperature Tc between 2600 and 4000 K), is highly suitable as a light source for, for example, interior lighting. With this lamp the perception is used to advantage that a good color rendition is possible when Na-halide is used as a filling component of a lamp and, when the lamp is in operation, there is a strong widening and reversal of the Na emission in the Na-D lines. This requires a high cold spot temperature Tkp in the discharge vessel of at least 1170 K (900xc2x0 C.). When the Na-D lines are reversed and widened, they assume in the spectrum the form of an emission band having two maximums mutually xcex94xcex apart.
The requirement of a large value of Tkp entails that the discharge vessel is relatively small, excludes the use of quartz or quartz glass for the wall of the discharge vessel and forces one to use ceramic for the wall of the discharge vessel.
In this description and these claims the ceramic wall is understood to mean both a wall of metal oxide such as, for example, sapphire or sintered polycrystalline Al2O3, and metal nitride, for example, AlN.
The filling of the discharge vessel contains besides Na and Tl, one or more rare-earth metals with which a desired value for the general color rendition index Raxe2x89xa780 and the color temperature Tc is realized. Rare-earth metals in this description and these claims are understood to mean the elements Sc, Y and the lanthanides.
A disadvantage of the known lamp is that under the influence of the rare-earth metals present during lamp operation there is corrosion of parts of the discharge vessel, more particularly, the wall. This finally results in a premature end of the lamp life. A further disadvantage of the known lamp is that also due to the relatively small dimensions of the discharge vessel, a relatively fast blackening of the wall of the discharge vessel occurs owing to deposition on the wall of W evaporated from the electrodes.
In a lamp according to the invention the ionizable filling also contains Ca and is free from rare-earth halides.
As a result of a surprisingly large spectral contribution of Ca both to the red and the blue, a value of Raxe2x89xa780 is realized for the general color rendition index and Tc up to 3500 K is realized for the color temperature. In addition, it surprisingly appears that formation of stable Ca aluminate compounds is eliminated and the Ca present causes a W-halide cycle to develop as a result of which also the blackening of the wall of the discharge vessel owing to the evaporation of W of the electrodes is strongly counteracted. A condition for the occurrence of the W-halide cycle is the presence in the discharge vessel of a small quantity of free oxygen. Generally, the quantity of free oxygen comes from contaminations occurring during the manufacture of the lamp and released therefrom when the lamp is in the operating state. It has also been established that oxygen is released from the ceramic wall material under the influence of reactions with filling components of the discharge vessel. In the case of too small a concentration, it will hardly be possible to maintain the W-halide cycle sufficiently during the operation of the lamp. In the case of too large a concentration there will be, inter alia, corrosion of the W-electrodes.
In a preferred embodiment, the discharge vessel contains an oxygen dispenser. This has the important advantage that oxygen is introduced into the discharge vessel in a controlled manner. Bearing in mind an accuracy of manufacture required for a proper operation of the lamp and consequent scaling down of contaminations, there is a large chance of too small a concentration with respect to the quantity of O2 that is released from contaminations. An additional advantage of the lamp according to the preferred embodiment is that dosaging during the life of the lamp becomes possible. In an advantageous embodiment of the lamp according to the invention, the oxygen dispenser contains CaO. CaO is advantageous in that by itself it forms part of the filling of the discharge vessel.
The filling of the discharge vessel can, in addition to Na and Tl, contain one or more metals, inter alia, for affecting the color properties of the lamp, for example, In. Besides the exclusion of rare-earth metals, a use of Ti, Zr and Hf is less suitable for the filling, because they form relatively stable oxides.
Experiments have shown that a value for xcex94xcex between 12 nm and 60 nm is desired for effecting good color properties of the lamp. With a value for Tkp in a range between 1200 K and 1300 K, a desired magnitude for xcex94xcex may generally be practicable, while also a maximum temperature of the wall of the discharge vessel up to 1450 K can be realized.